Heat-trapping layers with high electrical conductivity and low thermal conductivity confine heat inside switchable layers of heat-assisted resistance-switching (RS) cells, such as nonvolatile memory cells, so that switching processes involving Joule heating consume less electrical power. The same approach may reduce power consumption during activation of those types of selectors that require heat to activate (not all selectors use heat).
Resistance-switching NVM, also known as ReRAM or RRAM, is a possible replacement for flash memory and other charge-storage-based forms of nonvolatile memory. Resistance-switching cells repeatedly change their resistance between at least two distinguishable values in response to a signal, such as an applied electric, magnetic, thermal, chemical, optical, or combination stimulus. In the absence of the resistance-changing stimulus, and even in the absence of supplied power, the cell retains its last programmed resistance, making it usable as a static, or nonvolatile, memory element.
In the simplest case, the cell switches between two resistance values, a low-resistance state (LRS) and a high-resistance state (HRS). Such a cell can store one bit of data by assigning one state to “logic-zero” and the other to “logic-one.” The cell is written (its resistance is changed) by the application of a write signal (e.g., voltage, current, heat, light, etc.) that is at or above a write-threshold strength. The cell is read (its resistance is sensed without being changed) by applying a voltage (or current) that is below the write-threshold strength, measuring the output current (or voltage), and applying Ohm's law (R=V/I, where R is resistance, V is voltage, and I is current.
Cells that can be repeatedly switched between more than two resistance states have been demonstrated. Such cells may store multiple bits of data.
Although NVM is a leading application of resistance-switching cells, the cells may also be used for other switching applications; for example, as a threshold switch or a logic element that keeps its state when the device is powered down and powered back up.
Some non-limiting examples of resistance-switching modalities include redistribution of conductive metal atoms in an otherwise insulating medium, redox reactions that change the ionic content of the medium, varying the material phase or degree of crystallinity, or varying the ferromagnetic properties. Many types of resistance-switching designs and mechanisms have their own names and acronyms, and some writers treat particular schemata as different from what they term “ReRAM” or “RRAM.” For purposes herein, “resistance-switching,” “resistance-switchable,” and “RS” are interchangeable and refer to any repeatable or reversible resistance changes (i.e., they exclude irreversible changes such as the electrical breakdown that occurs in a fuse or anti-fuse). “RS-NVM” means non-volatile memory based on repeatedly or reversibly induced resistance changes in selected individual memory elements or cells.
Reliable and responsive materials are critical to RS-NVM performance. However, sometimes there are trade-offs between responsivity and reliability because sensitive materials may be sensitive to other stimuli besides the switching signal. Thermal issues are often a cause for concern, more so as memory-cell density increases and memory arrays are placed closer to processor logic arrays. Yet some types of switching make use of heat. Heat may be the dominant switching stimulus in some phase-change and thermochemical switching. In addition, some types of electrical switching in redox, valency-change, crystallinity-change, and metal-migration cells may be assisted by localized heat; that is, switching at a higher temperature consumes less electrical power or has some other advantage.
Therefore, a need exists for controlling thermal conditions in RS-NVM cells and their near neighboring components such as diodes, transistors, other switches, and other cells in the RS-NVM array. Heating is preferably sufficient where it is necessary or beneficial to the intended function, yet preferably it is not so excessive or uncontrolled that performance of other components, or of the device as a whole, is compromised. Operating at lower power reduces the amount of heat dissipated.